Structures requiring high strength-to-weight ratios, such as aircraft and spacecraft wing, tail, and fuselage assemblies, have traditionally been constructed primarily with monolithic metallic materials, such as aluminum-alloy sheets and plates. A current trend is to make such structures from individually designed elements that are engineered to have the required structural properties with reduced weight. Some examples of such individually designed elements include composite materials made of carbon or other fibers in a cured polymeric matrix. For structures subjected to higher temperatures, these individually designed elements include hollow superplastically-formed/diffusion-bonded (SPF/DB) titanium-alloy structures and hollow, metallic honeycomb structures. These structural elements are specifically engineered to take advantage of both the structural characteristics arising from the arrangement of the various components following assembly and also the mechanical and physical properties of the materials that form the various structural elements.
As with more conventional structures, these advanced structural elements must be joined to each other and to other structures to form the resulting assembly, such as an aircraft or spacecraft wing, tail or fuselage assembly. For some low-temperature applications, adhesives can sometimes be used. It has been found to be difficult, however, to join many advanced structures with conventional fasteners since the clamping force applied by the fastener must be carefully limited. For other low-temperature applications as well as high-temperature applications, fasteners such as rivets, bolts, clips, and the like are used to join the various structural elements.
Conventional fasteners usually rely upon the application of compressive preloads to hold the fastened structures together. Advanced structures, such as hollow SPF/DB or honeycomb structures having a porous or low-density structural core sandwiched between face sheets, may be crushed, however, by the application of excessively high compressive fastening preloads, thereby altering the resulting shape of the assembly in a manner that is generally unsuitable for the intended application.
There have been proposed various advanced fasteners for use with the advanced structures to limit the compressive preloads applied by the fastener to the resulting assembly. Although these advanced fasteners have met with varying degrees of success, each of these proposed fasteners have been found to be deficient or limited in some respect.
For example, Hi-Lok type fastener collars have been developed by Hi-Shear Corporation that include torque limiting features for protecting the structural elements from excessive compressive preloads. As known to those skilled in the art, a Hi-Lok type fastener includes a threaded pin that is driven through a pair of aligned holes that have previously been formed in the structural elements to be joined. The pin of the Hi-Lok type fastener is sized to be interference-fit within the holes. A Hi-Lok type fastener also includes a nut or collar installed from the opposite or back side of the resulting assembly. According to some designs, the frangible collar of a Hi-Lok type fastener includes a drive section to which torque is applied to advance the collar upon the threaded end portion of the pin. Upon achieving the prescribed seating torque, the drive section of the collar shears from the remainder of the collar to prevent further torque from being applied to the resulting assembly. As described, however, a Hi-Lok type fastener requires access to the back side of the resulting structure in order to tighten the collar upon the threaded end portion of the pin. In many instances, however, the back side of the resulting structure is blind, that is the back side of the resulting structure will be inaccessible, at least during the latter stages of assembly at which time the fastener is to be installed. As such, a Hi-Lok type fastener is unsuited for the assembly of structural elements having a blind side.
In addition, advanced structures often do not have precisely reproducible thickness due to the processes by which the structures are fabricated, However, the advanced fasteners that are available do not account for the variations in thickness that may be found in such advanced structures. As such, the advanced fasteners must therefore be individually tailored to compensate for the variations in thickness of each one of the structures. As will be readily appreciated, this individual tailoring of the fasteners is a time-consuming, and expensive process.
Therefore, there is a need for an advanced fastener for joining advanced structures that are susceptible to crushing damage in a manner which achieves acceptable fastening performance without damaging the structure. In addition, there is a need for an advanced fastener which can be installed in blind applications and which has a relatively smooth head following installation. There is also need for an advanced fastener that securely joins structural elements which permits variations in the thickness of structural elements.